If you're cycling with fish in the tank you're going to need to do water changes to keep the nitrite levels down otherwise they can die from Nitrite poisoning.
This is one of the reasons why cycling with fish can take longer as you have to keep the levels dilute enough for the fish to survive in thus the bacteria have less to feed upon.
How big is the tank, smaller tanks will need more frequent water changes as there is less volume to dilute the toxins?
What are your current ammonia/nitrite levels???
